Disc drive storage capacity has been increasing dramatically recently. This means that the data tracks per inch (TPI) on a disc surface increases in order to pack more data on a given surface area of the disc. One of the challenges facing disc drive designers is data integrity. Data integrity problems become more pronounced and difficult to handle as the tolerances of tracks and between tracks become tighter. When the track pitch falls below 200 μin, maintaining the data heads within the acceptable limits of track center becomes very difficult. A shock of fairly short duration (less than 5 msec) can knock the head far off a track. External shock or vibration, during a read operation, may cause a ‘soft data error’. The desired data can be recovered from read retries and data throughput is then affected. If the head is knocked off a track while the data is being written, then the old data may be overwritten and become permanently lost. This is commonly known as a ‘hard data error’. To ensure data integrity, the write gate is turned off to prevent write current from reaching the head and thus the hard data error from occurring when an excessive shock is sensed.
A shock sensor mounted a drive can sense the incoming shock well before the position error signal (PES) goes beyond the write fault threshold. This is because the head is under closed loop control and the control system will try to keep the head on track via closed loop control when a shock comes in. A shock sensor is an analog device. It can continuously monitor the external shock. When a shock sensor detects a shock, it will turn off the write gate to prevent permanent data being lost on the disk. Disadvantages of the use of dedicated shock sensors are: (1) additional cost; and (2) potentially sensing the wrong trigger, which may lower the data throughput.
More accurate detection can be accomplished using two axis shock sensors. Other shock detection schemes using hardware have also been proposed such as utilizing the back electromotive force (EMF) signal of the actuator to sense an incoming shock. Another approach is to use staggered servo wedges on different heads to achieve a higher sampling rate on head position, and thus, a faster detection of a shock event. However, in this latter case, the scheme cannot be applied to a single head drive, and it requires 2 read channels in the servo control system that, in turn, increases the cost.
In many low cost drive designs external shock is sensed from the Position Error Signal (PES) as a read head passes over a servo sector on a disc surface. Due to the limitations such as drive format efficiency and microprocessor bandwidth in low cost drives, the number of servo sectors on a track does not grow as fast as does the number of tracks per inch (TPI). In a higher TPI drive, therefore, the sector-to-sector transition time is relatively longer. In these drives, when a shock event occurs, a head may go further off of a track before the control system senses a shock from the PES. Currently, the write fault detection in a typical conventional drive is based on the following conditions:PES(k)>WriteFaultThreshold or  (i)2*PES(k)−PES(k−1)=PES(k)+[PES(k)−PES(k−1)]=Predict—PES(k+1)>1.4*WriteFaultThreshold  (ii)where “k” is a track sector index. “WriteFaultThreshold” is for example, set to 15% of track pitch (TP). The control system checks the value of the current sector PES and the predicted PES for the next sector. If they are greater than the thresholds, the control scheme will turn off the write gate. In this conventional scheme, the PES is delayed, compared to the time when shock comes in, due to the closed loop control. So, the write fault detection may also be delayed. It has been observed that sometimes the actual write fault detection is a bit late. In other words, at the time the servo gate is actually switched off, the PES is greater than the WriteFaultThreshold already. In other words, the head position is further off track than desired when the servo gate is actually switched off, which could permit write errors to occur. Accordingly there is a need for a shock event detection system that senses a shock event sooner and thus precludes such write errors from occurring.